1. Field of the Invention
This invention relates to compositions and processes for removing residues that accumulate, during repeated use, on the surfaces of metal molds used for molding of plastics from powder or sheet form into coherent, macroscopic, shaped solid objects. The invention particularly relates to cleaning such molds used for poly{vinyl chloride} (hereinafter usually abbreviated as "PVC") and similar plastics that require relatively high molding temperatures.
2. Discussion of Related Art
At least the molding surfaces of metal molds for plastics fabrication are usually constituted of nickel, chromium, and/or stainless steel with a high content of nickel and/or chromium. Furthermore, these molding surfaces when new have a specific surface texture that it is desired to impart to the plastic objects being molded with them. For example, a mold surface may be highly polished to produce a smooth and glossy surface on objects molded with it, or it may have a microtexture deliberately designed to minimize gloss, reflectivity, and/or slippperiness, as is true for many of the plastic objects molded for use in automobiles, where avoidance of any sun glare is desired for such items as dashboards and a non-slippery grip is desired for steering wheels.
It is known that, when metal molds are used repeatedly to mold plastic objects, solid residues remain on the metal mold surface after the objects molded have been discharged from the molds. Although the residues are generally presumed to consist of constituents of the plastic molding composition and/or of products of chemical reaction among such constituents or among the constituents and the metal itself, the chemical nature of the residues is not usually known in detail.
Eventually the accumulation of residues reaches a sufficient volume on some part of the molding surface that the plastic objects molded therewith no longer satisfy surface texture requirements, and then the residues must be removed before use of the mold surface bearing such accumulated residue can be effectively continued. In many instances in commercial practice, no completely satisfactory chemical cleaner to remove such molding residues has been known, and the only effective method of cleaning has been mechanical, usually blasting with grit or like materials. Such blasting normally requires removing the molding surface from its place of normal use to another area where blasting is performed, and thereby necessitates the substitution of an alternative molding surface if production of molded plastic objects is to be continued, as is normally preferred, while the blasting is carried out. Furthermore, the blasting itself can eventually damage the mold surface. Thus the entire process is inefficient compared to the kind of chemical cleaning which is available for many other types of soiled metal objects.